The present disclosure relates to a receiving apparatus, a receiving method, and a receiving system. More particularly, the disclosure relates to a receiving apparatus, a receiving method, and a receiving system each of which is capable of preventing deterioration of estimate precision of transmission path characteristics as characteristics of a transmission path for an OFDM signal.
The terrestrial digital broadcasting or the like adopts an Orthogonal Frequency Division Multiplexing (OFDM) system as a modulation system for modulating data (signal).
In the OFDM system, digital modulation such as Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) is carried out. In such a digital modulation, a large number of orthogonal sub-carriers are provided within a transmission band, and predetermined pieces of data are allocated to amplitudes and phases of the sub-carriers, respectively.
In the OFDM system, since the transmission band is divided with a large number of sub-carriers, the band per one (one wave) sub-carrier becomes narrow, and thus a modulation rate becomes low. However, an overall transmission rate (of the entire sub-carriers) is not changed from that in the case of a modulation system in the related art.
As has been described, since in the OFDM system, the predetermined pieces of data are allocated to plural sub-carriers, respectively, the modulation can be carried out by using an Inverse Fast Fourier Transfer (IFFT) arithmetic operation for carrying out inverse Fourier transfer. In addition, the demodulation of the OFDM signal obtained through the modulation can be carried out by using a Fast Fourier Transfer (FFT) arithmetic operation for carrying out Fourier transform.
Therefore, a transmitting apparatus which transmits the OFDM signal can be configured by using a circuit for carrying out the IFFT arithmetic operation. Also, a transmitting apparatus which receives the OFDM signal can be configured by using a circuit for carrying out the FFT arithmetic operation.
In addition, in the OFDM system, provision of signal intervals called guard intervals which will be described later makes it possible to enhance the resistance against a multi-path. In addition, in the OFDM system, pilot signals as known signals (signals known on the receiving apparatus side) are discretely inserted into a time direction or a frequency direction. The receiving apparatus utilizes the pilot signals in synchronization, and estimation of the transmission path characteristics.
The OFDM system is adopted in the terrestrial digital broadcasting or the like on which an influence of multi-path interference is strongly exerted because the OFDM system has the strong resistance against the multi-path. Digital Video Broadcasting-Terrestrial (DVB-T), Integrated Services Digital Broadcasting-Terrestrial (ISDB-T), and the like, for example, are known as the standards for the terrestrial digital broadcasting adopting the OFDM system.
In the OFDM system, the data is transmitted in units called OFDM symbols.
FIG. 1 is a diagram showing the OFDM symbols.
The OFDM symbol is composed of an effective symbol and a guard interval. In this case, the effective symbol is a signal period of time for which the IFFT is carried out in a phase of the modulation. Also, in the guard interval, a waveform of a part of the second half of the effective symbol is copied in a head of the effective symbol as it is.
When let Tu sec. to be a length of the effective symbol of the OFDM symbol, that is, a length of the effective symbol as a length not containing therein the guard interval, and let Fc Hz to be an interval between the sub-carriers of the OFDM, a relationship of Fc=1/Tu holds.
In the OFDM system, as shown in FIG. 1, the guard interval is provided in the head of the OFDM symbol, thereby enhancing the resistance against the multi-path.
In the terrestrial digital broadcasting, a unit called an OFDM transmission frame is defined, and the OFDM transmission frame is composed of plural OFDM symbols.
For example, in the ISDB-T standard, one OFDM transmission frame is composed of 204 OFDM symbols. Also, a position where the pilot signal is inserted is previously determined with the OFDM transmission frame as a unit.
In the OFDM system in which the modulation of the QAM system is used in the modulation of the sub-carriers, in a phase of the transmission, an influence which differs every sub-carrier is exerted on the amplitude and phase of the sub-carrier of the OFDM signal obtained by subjecting the data to the OFDM by the multi-path or the like.
For this reason, the receiving apparatus carries out distortion correction for equalizing the OFDM signal received from the transmitting apparatus in such a way that an amplitude and a phase of a sub-carrier of the OFDM signal received from the transmitting apparatus become equal to an amplitude and a phase of the sub-carrier of the OFDM signal transmitted from the transmitting apparatus, respectively.
That is to say, in the OFDM system, the transmitting apparatus discretely inserts (the transmission symbols of) the known pilot signals whose amplitudes and phases are previously determined as the transmission symbols (sub-carriers) composing the OFDM symbols, respectively. Also, the receiving apparatus estimates the transmission path characteristics as the characteristics (frequency characteristics) of the transmission path in accordance with the amplitudes and phases of the pilot signals, and carries out the distortion correction for the OFDM signal by using the transmission path characteristic data representing the transmission path characteristics thus estimated.
FIG. 2 is a diagram showing an example of an arrangement pattern of (the transmission symbols of) the pilot signals within the OFDM symbols.
It is noted that in FIG. 2 (to which FIGS. 5, 6, 14, and 15 as will be described later are also similar), an axis of abscissa represents a sub-carrier number based on which the sub-carrier of the OFDM signal is specified, and an axis of ordinate represents an OFDM symbol number based on which the OFDM symbol of the OFDM signal is specified.
The sub-carrier number corresponds to a frequency, and the OFDM symbol number corresponds to time.
In FIG. 2, each of circle marks (an open circle mark and a black circle mark) represents either the sub-carrier of the OFDM signal, or a transmission symbol (a symbol on IQ constellation as data for modulation of the sub-carrier on a transmitting apparatus side) composing an OFDM symbol.
In addition, in FIG. 2, the black circle mark represents a transmission symbol of the pilot signal.
In FIG. 2, the transmission symbols of the pilot signals are arranged in plural positions previously determined, respectively, in the OFDM signal.
That is to say, in FIG. 2, (the transmission symbol of) the pilot signal is arranged every four OFDM symbols (OFDM symbol numbers) in the time direction. Also, (the transmission symbol of) the pilot signal is arranged every twelve sub-carriers (sub-carrier numbers) in the frequency direction.
With regard to the pilot signal, there are a pilot signal called a Scattered Pilot (SP) and a pilot signal called a Continual Pilot (CP).
The SP is periodically arranged every predetermined number of sub-carriers (transmission symbols), and is used to estimate the transmission characteristics. The CPs are arranged in the sub-carriers having the same frequency (a frequency previously determined), respectively.
FIG. 2 shows an example of the arrangement pattern of the SPs.
In the DVB-T and the ISDB-T, the arrangement pattern of the SPs is fixed to one kind of arrangement pattern. However, in Digital Video Broadcasting-Sound Generation Terrestrial (DVB-T.2), plural arrangement patterns (eight kinds of arrangement patterns) are determined as the arrangement pattern of the SPs. Also, the SPs are arranged in accordance with one of the plural arrangement patterns.
FIG. 3 is a block diagram showing a configuration of an example of an existing receiving apparatus for receiving the OFDM signal.
In FIG. 3, the receiving apparatus includes an antenna 11, a tuner 12, a Band-Pass Filter (BPS) 13, an Analog/Digital (A/D) conversion portion 14, an orthogonally demodulating portion 15, an offset correcting portion 16, an FFT control portion 17, an FFT portion 18, a transmission characteristics estimating section 19, a transmission path distortion correcting portion 20, and an error correcting portion 21.
The antenna 11 receives thereat a broadcasting wave of the OFDM signal which is transmitted (broadcasted) from a transmitting apparatus of a broadcasting station (not shown), converts the broadcasting wave thus received thereat into a Radio Frequency (RF) signal, and supplies the resulting RF signal to the tuner 12.
The tuner 12 extracts a signal having a predetermined frequency from the RF signal supplied thereto from the antenna 11, frequency-converts the signal having the predetermined frequency thus extracted into an Intermediate Frequency (IF) signal, and supplies the resulting IF signal to the BPF 13.
The BPF 13 filters the IF signal supplied thereto from the tuner 12, and supplies the resulting IF signal as an analog signal to the A/D conversion portion 14. The A/D conversion portion 14 subjects the IF signal as the analog signal supplied thereto from the BPF 13 to the A/D conversion, and supplies the resulting IF signal as a digital signal to the orthogonally demodulating portion 15.
The orthogonally demodulating portion 15 orthogonally demodulates the IF signal as the digital signal supplied thereto from the A/D conversion portion 14 by using a carrier having a predetermined frequency (carrier frequency) and outputs the resulting OFDM signal in a base band.
Here, the OFDM signal which is outputted by the orthogonally demodulating portion 15 is a signal in a time region before an FFT arithmetic operation is carried out (right after a transmission symbol on an IQ constellation has been subjected to the IFFT arithmetic operation on the transmitting apparatus side). Hereinafter, the OFDM signal is referred to as “an OFDM time region signal” as well.
The OFDM time region signal is a complex signal expressed by a complex number containing therein a real axis component (In Phase (I) component) and an imaginary axis component (Quadrature Phase (Q) component).
The OFDM time region signal is supplied from the orthogonally demodulating portion 15 to the offset correcting portion 16.
The offset correcting portion 16 carries out correction for a sampling offset (a shift in a sampling timing) in the A/D conversion portion 104 with the OFDM time region signal supplied thereto from the orthogonally demodulating portion 15 as an object. Also, the offset correcting portion 16 carries out correction for an offset of a frequency of a carrier (a shift with respect to a frequency of a carrier used in the transmitting portion) in the orthogonally demodulating portion 15.
In addition, the offset correcting portion 16, for example, carries out filtering for removing the same channel interference and an adjacent channel interference, or the like as may be necessary.
The OFDM time region signal which has been processed in the offset correcting portion 16 is supplied to each of the FFT control portion 17 and the FFT portion 18.
The FFT control portion 17, for example, carries out an arithmetic operation for a correlation between the OFDM time region signals supplied thereto from the offset correcting portion 16, thereby detecting a start position of an FFT interval (FFT start position) as an interval of the OFDM time region signals as an object of the FFT arithmetic operation in the FFT portion 18. Also, the FFT control portion 17 supplies a timing signal representing the FFT start position to the FFT portion 18.
The FFT portion 18 carries out the FFT arithmetic operation for the OFDM time region signal supplied thereto from the offset correcting portion 16 in accordance with the timing signal supplied thereto from the FFT control portion 17. Carrying out the FFT arithmetic operation for the OFDM time region signal results in that data transmitted on the sub-carrier, that is, an OFDM signal representing a transmission symbol on an IQ constellation is obtained.
Here, the OFDM signal obtained through the FFT arithmetic operation for the OFDM time region signal is a signal in a frequency region, and is hereinafter referred to as “an OFDM frequency region signal” as well.
The FFT portion 18 supplies the OFDM frequency region signal obtained through the FFT arithmetic operation to each of the transmission path characteristics estimating section 19 and the transmission path distortion correcting portion 20.
The transmission path characteristics estimating section 19 estimates the transmission path characteristics for the sub-carriers (transmission symbols) of the OFDM signal by using the pilot signals SPs arranged in the manner as shown in FIG. 2 in the OFDM frequency region signal supplied thereto from the FFT portion 18. Also, the transmission path characteristics estimating section 19 supplies the transmission path characteristic data as an estimate value of the transmission path characteristics to the transmission path distortion correcting portion 20.
The transmission path distortion correcting portion 20 carries out the distortion correction for correcting distortions of a amplitude and a phase of the sub-carrier of the OFDM signal caused in the transmission path by using the transmission path characteristic data supplied thereto from the transmission path characteristics estimating section 19 with the OFDM frequency region signal supplied thereto from the FFT portion 18 as an object. In this case, for example, the transmission path distortion correcting portion 20 carries out the distortion correction for the OFDM frequency region signal by, for example, dividing the OFDM frequency region signal by the transmission path characteristic data. Also, the transmission path distortion correcting portion 20 supplies the OFDM frequency region signal obtained after completion of the distortion correction to the error correcting portion 21.
The error correcting portion 21 executes necessary error correcting processing with the OFDM frequency region signal supplied thereto from the transmission path distortion correcting portion 20 as an object. That is, for example, the error correcting portion 21 carries out de-interleave, de-puncture, Viterbi decoding, diffusion signal removal, Low Density Parity Check (LDPC) decoding, and Reed-Solomon (RS) decoding, and outputs the resulting decoded data.
FIG. 4 is a block diagram showing an example of a configuration of the transmission path characteristics estimating section 19 shown in FIG. 3.
In FIG. 4, the transmission path characteristics estimating section 19 includes a pilot extracting portion 31, a reference signal generating portion 32, an estimating portion 33, a time direction interpolating portion 34, and a frequency direction interpolating part 35. In addition, the frequency direction interpolating part 35 includes a position adjusting portion 36, a filter center controlling portion 37, an up-sampling portion 38, and an interpolation filter 39.
The OFDM frequency region signal which has been supplied from the FFT portion 18 to the transmission path characteristics estimating section 19 is in turn supplied to the pilot extracting portion 31.
The pilot extracting portion 31 extracts the transmission symbols of the SPs, for example, arranged in the manner as shown in FIG. 2 from the OFDM frequency region signal supplied thereto from the FFT portion 18, and supplies the transmission symbols of the SPs thus extracted to the estimating portion 33.
The reference signal generating portion 32 generates (the transmission symbol of) the same SP as that contained in the OFDM signal by the transmitting apparatus. Also, the reference signal generating portion 32 supplies (the transmission symbol of) the SP thus generated as a reference signal becoming a reference in estimation of the transmission path characteristics for the transmission symbols of the pilot signals contained in the OFDM frequency region signal to the estimating portion 33.
Here, in the case of the ISDB-T standard and the DVB-T standard, the transmission symbol of the pilot signal is a signal obtained by subjecting predetermined data to Binary Phase Shift Keying (BPSK) modulation. Then, the reference signal generating portion 32 generates the transmission symbol obtained by subjecting the predetermined data to the BPSK modulation, and supplies the transmission symbol thus generated as a reference signal to the estimating portion 33.
The estimating portion 33 estimates the transmission path characteristics for the transmission symbol of the SP (hereinafter referred to as “the SP transmission path characteristics” as well) by dividing the transmission symbol of the SP supplied thereto from the pilot extracting portion 31 by the reference signal supplied thereto from the reference signal generating portion 32. Also, the estimating portion 33 supplies SP transmission path characteristic data as an estimate value of the transmission path characteristics to the time direction interpolating portion 34.
Here, the distortion of the OFDM signal caused by the transmission path (the distortion due to the multi-path or the like) comes to form multiplication for the OFDM signal. Therefore, the estimation of the SP transmission path characteristics as a distortion component of the OFDM signal caused by the transmission path can be carried out by dividing the transmission symbol of the SP supplied from the pilot extracting portion 31 by the reference signal.
The time direction interpolating portion 34 carries out the interpolation for the time direction by using the SP transmission path characteristic data supplied thereto from the estimating portion 33 in a symbol number direction (in a time direction). Also, the time direction interpolating portion 34 supplies time direction interpolation characteristic data obtained through the interpolation to the frequency direction interpolating part 35.
The frequency direction interpolating part 35 carries out filtering for interpolating the time direction interpolation characteristic data supplied thereto from the time direction interpolating portion 34 in the frequency direction. Thereby, the frequency direction interpolating part 35 supplies transmission path characteristic data (hereinafter referred to as “frequency direction interpolation characteristic data” as well) as the estimate value for estimation of the transmission path characteristics in which the interpolation in the frequency direction is carried out, in a word, the transmission path characteristics for the transmission symbols (sub-carriers) of the OFDM symbols to the transmission path distortion correcting portion 20.
That is to say, in the frequency direction interpolating part 35, the time direction interpolation characteristic data supplied from the time direction interpolating portion 34 is supplied to each of the position adjusting portion 36 and the filter center controlling portion 37.
The position adjusting portion 36 adjusts (rotates) a phase of the time direction interpolation characteristics supplied thereto from the time direction interpolating portion 34 in accordance with optimal position information supplied from the filter center controlling portion 37. Also, the position adjusting portion 36 supplies time direction interpolation characteristic data obtained through the adjustment of the phase of the time direction interpolation characteristics to the up-sampling portion 38.
On the other hand, the time direction interpolation characteristic data is supplied from the time direction interpolating portion 34 to the filter center controlling portion 37. In addition thereto, the OFDM frequency region signal is also supplied from the FFT portion 18 to the filter center controlling portion 37.
The filter center controlling portion 37 carries out filtering for the time direction interpolation characteristic data supplied thereto from the time direction interpolating portion 34, that is, (time direction interpolation characteristic data obtained by carrying out the interpolation in the time direction for) SP transmission path characteristic data obtained by using the SP by using an interpolation filter (not shown) while a position of a filter center is adjusted. Thereby, the filter center controlling portion 37 obtains transmission path characteristic data (frequency direction interpolation characteristic data) as an estimate value of the transmission path characteristics for the transmission symbols of the OFDM frequency direction region signal.
In addition, the filter center controlling portion 37 carries out distortion correction for (the transmission symbols of) the OFDM frequency region signal supplied thereto from the FFT portion 18 by using the transmission path characteristic data. Also, the filter center controlling portion 37 obtains a signal quality of the OFDM frequency region signal after completion of the distortion correction with the predetermined symbol as an object.
Here, in the filter center controlling portion 37, for example, with regard to the ISDB-T, information on the signal quality is obtained with the transmission symbol of Transmission and Multiplexing Configuration Control (TMCC)/Auxiliary Channel (AC) as an object. Also, with reference to the DVB-T, information on the signal quality is obtained with the transmission symbol of Transmission Parameters Signalling (TPS) as an object.
With regard to the signal quality, it is possible to adopt a distance (on the IQ constellation) between the transmission symbol after completion of the distortion correction, for example, corresponding to an amount of noise in the transmission symbol after completion of the distortion correction, and a hard decision value of that transmission symbol. In this case, the signal quality represents that the smaller the value, the better the quality.
The filter center controlling portion 37 obtains information on the signal quality of the OFDM frequency region signal after completion of the distortion correction while the position of the filter center (of the interpolation filter (not shown)) is adjusted, and obtains information on a position of the filter center in which the signal quality becomes optimal (hereinafter referred to as “an optimal position” as well).
Also, the filter center controlling portion 37 controls the position of the filter center of the interpolation filter 39 in such a way that the position of the filter center of the interpolation filter 39 becomes the optimal position.
That is to say, the filter center controlling portion 37 supplies optimal position information representing the optimal position to the position adjusting portion 36.
As has been described, the position adjusting portion 36 adjusts the phase of the time direction interpolation characteristic data supplied thereto from the time direction interpolating portion 34 in accordance with the optimal position information supplied thereto from the filter center controlling portion 37, that is, rotates the time direction interpolation characteristic data on the IQ constellation. Thereby, the position of the filter center in the phase of the filtering for the time direction interpolation characteristic data in the interpolation filter 39 which will be described later is made to agree with the position represented by the optimal position information.
In the manner as described above, the up-sampling portion 38 interpolates zeros the number of which is equal to the number of transmission symbols for which no estimate value of the transmission path characteristics is obtained when viewed in the frequency direction, for example, two zeros between the sample values of the time direction interpolation characteristic data after completion of the adjustment of the phase obtained in the position adjusting portion 36. Thereby, the up-sampling portion 38 generates time direction interpolation characteristic data in which an amount of data (the number of sample values) is made three times as large as original one, and supplies the time direction interpolation characteristic data thus generated to the interpolation filter 39.
The interpolation filter 39 is a Low-Pass Filter (LPF) for carrying out the filtering for the interpolation in the frequency direction. Thus, the interpolation filter 39 carries out the filtering for the time direction interpolation characteristic data supplied thereto from the up-sampling portion 38.
The filtering carried out by the interpolation filter 39 results in removal of a repetitive component generated in the time direction interpolation characteristic data by the interpolation of zeros in the up-sampling portion 38. Thus, there is obtained frequency direction interpolation characteristic data as an estimate value of the transmission path characteristics for which the interpolation in the frequency direction is carried out, that is, the transmission path characteristics for the transmission symbols (sub-carriers) of the OFDM symbols.
The frequency direction interpolation characteristic data which has been obtained in the interpolation filter 39 in the manner as described above is supplied in turn as transmission path characteristic data used for the distortion correction of the OFDM signal to the transmission path distortion correcting portion 20.
Here, the transmission path characteristics estimating section 19 supplies not only the transmission path characteristic data obtained in the interpolation filter 39, but also the optimal position information obtained in the filter center controlling portion 37 to the transmission path distortion correcting portion 20.
The transmission path distortion correcting portion 20 adjusts (rotates) the phase of the OFDM frequency region signal from the FFT portion 18 in accordance with the optimal position information supplied thereto from the transmission path characteristics estimating section 19. After that, the transmission path distortion correcting portion 20 divides the OFDM frequency region signal by the transmission path characteristic data supplied thereto from the transmission path characteristics estimating section 19, thereby carrying out the distortion correction for the OFDM frequency region signal.
FIG. 5 is a diagram explaining the time direction interpolation characteristic data as the estimate values of the transmission path characteristics which the time direction interpolating portion 34 shown in FIG. 4 obtains by using the transmission path characteristic data (SP transmission path characteristic data) on the positions of the SPs shown in FIG. 2, and for which the interpolation in the time direction is carried out.
In FIG. 5, circle marks (an open circle mark and a shaded circle mark) represent the transmission symbols (sub-carriers) of the OFDM signal.
In addition, in FIG. 5, the shaded circle mark represents the transmission symbol in which the estimate value of the transmission path characteristics is contained in the time direction interpolation characteristic data (a sample value of the time direction interpolation characteristic data exists).
According to the interpolation in the time direction carried out in the time direction interpolating portion 34, as shown in FIG. 5, the estimate value of the transmission path characteristics for each three transmission symbols (in the frequency direction) can be obtained from the OFDM signal in which the SPs are arranged in the manner as shown in FIG. 2 with respect to each of the OFDM symbols.
FIG. 6 is a diagram explaining the frequency direction interpolation characteristic data as the estimate values of the transmission path characteristics which are obtained by the frequency direction interpolating part 35 by using the time direction interpolation characteristic data as the estimate values of the transmission path characteristics of the transmission symbols each indicated by a shaded circle mark in FIG. 5 and for which the interpolation in the frequency direction is carried out.
The frequency direction interpolating part 35 carries out the interpolation for the time direction interpolation characteristic data in which the estimate value of the transmission path characteristics is obtained every three transmission symbols in the sub-carrier number direction (in the frequency direction). Thereby, the frequency direction interpolating part 35 obtains frequency direction interpolation characteristic data as the estimate values of the transmission path characteristics of the respective transmission symbols of the OFDM symbols. In this case, the frequency direction interpolation characteristic data is surrounded with a shaded rectangle shown in FIG. 6.
That is to say, in the frequency direction interpolating part 35, the position adjusting portion 36 adjusts the phase of the time direction interpolation characteristic data supplied thereto from the time direction interpolating portion 34 in accordance with the optimal position information supplied thereto from the filter center controlling portion 37. Also, the position adjusting portion 36 supplies the time direction interpolation characteristic data after completion of the phase adjustment to the up-sampling portion 38.
The up-sampling portion 38 interpolates two zeros between the sample values of the time direction interpolation characteristic data (refer to FIG. 5) supplied thereto from the position adjusting portion 36 to generate the time direction interpolation characteristic data in which the amount of data is made three times as large as original one, and supplies the time direction interpolation characteristic data thus generated to the interpolation filter 39.
That is to say, as shown in FIG. 5, the time direction interpolation characteristic data supplied from the position adjusting portion 36 to the up-sampling portion 38 has an arrangement pattern (system) in which the estimation value of the transmission path characteristics is arranged every three transmission symbols when viewed in the frequency direction.
Therefore, with regard to the time direction interpolation characteristic data supplied from the position adjusting portion 36 to the up-sampling portion 38, only the two transmission symbols for each of which the transmission path characteristics have not been estimated exist between the transmission symbols for each of which the transmission path characteristics have been estimated when viewed in the frequency direction. For this reason, the up-sampling portion 38 interpolates the two zeros each becoming a sample point of the transmission path characteristics for the two transmission symbols for each of which the transmission path characteristics have not been estimated.
As has been described, the number of zeros interpolated in the up-sampling portion 38 differs depending on every how many transmission symbols when viewed in the frequency direction, the estimate values of the transmission path characteristics are arranged in the system which the time direction interpolation characteristic data obtained in the time direction interpolating portion 34 has.
As has been described, when in the up-sampling portion 38, the two zeros are interpolated between the sample values of the time direction interpolation characteristic data supplied from the position adjusting portion 36, time direction interpolation characteristic data (hereinafter referred to as “0 value interpolation characteristic data” as well) obtained from the interpolation result contains therein the repetitive component in the time region.
That is to say, the time direction interpolation characteristic data is data obtained from the OFDM frequency region signal, and is also data in the frequency region.
Also, the time direction interpolation characteristic data, and the 0 value interpolation characteristic data obtained by interpolating zeros in the time direction interpolation characteristic data are the same signal in terms of an analog signal. Therefore, data in the time region of the time direction interpolation characteristic data, and data in the time region of the 0 value interpolation characteristic data become data having the same frequency components.
In addition, the time direction interpolation characteristic data has the system of the estimate values of the transmission path characteristics for each three transmission symbols. The interval between the transmission symbols (sub-carriers) in the frequency direction, as described above, is expressed by Fc=1/Tu Hz. Therefore, an interval between the sample values (estimate values) of the time direction interpolation characteristic data as the system of the estimate value of the transmission path characteristics for each three transmission symbols (in the frequency direction) is expressed by 3Fc=3/Tu Hz.
Therefore, an interval between the sample values of the 0 value interpolation characteristic data obtained by interpolating the two zeros between the sample values of the time direction interpolation characteristic data is expressed by Fc=1/Tu Hz.
The time direction interpolation characteristic data in which the interval between the sample values is expressed by 3Fc=3/Tu Hz is data in which 1/3Fc=Tu/3 sec. is set as one period in the time region.
In addition, the 0 value interpolation characteristic data in which the interval between the sample values is expressed by Fc=1/Tu Hz is data in which 1/Fc=Tu sec. is set as one period in the time region, that is, data in which a period of time which is three times as long as the period of the time direction interpolation characteristic data is set as one period.
As has been described, the data in the time region of the 0 value interpolation characteristic data which contains therein the same frequency components as those of the data in the time region of the time direction interpolation characteristic data, and in which the period of time which is three times as long as the period of the data in the time region of the time direction interpolation characteristic data is set as one period is obtained by repeating the data in the time region of the time direction interpolation characteristic data three times.
That is to say, FIG. 7 shows the data in the time region of the 0 value interpolation characteristic data.
It is noted that in the following description, for ease of a description, it is assumed that the multi-path is composed of two paths (consisting of a main path and one echo) (two wave environment).
In FIG. 7 (to which FIG. 10 as will be described later is also similar), an axis of abscissa represents time, and an axis of ordinate represents a power level of a path (OFDM signal).
(The data in the time region of) The 0 value interpolation characteristic data having a period of Tu sec. is obtained by repeating the multi-path corresponding to (the data in the time region of) the time direction interpolation characteristic data having a period of Tu/3 sec. three times.
Now, in the 0 value interpolation characteristic data, of the multi-paths corresponding to the time direction interpolation characteristic data which is repeated three times, the second round of the multi-path (center multi-path) (indicated by slanting lines in FIG. 7) is assumed to be a desired multi-path which is extracted as the frequency direction interpolation characteristic data. In this case, for the purpose of obtaining the desired multi-path corresponding to the frequency direction interpolation characteristic data, it is necessary to remove all of other multi-paths.
Then, the interpolation filter 39 (refer to FIG. 4) removes all of the multi-paths other than the desired multi-path by filtering the 0 value interpolation characteristic data, thereby extracting the desired multi-path corresponding to the frequency direction interpolation characteristic data.
Note that, the 0 value interpolation characteristic data is the data in the frequency region. Thus, the filtering for the 0 value interpolation characteristic data in the interpolation filter 39 becomes the convolution of a filter coefficient of the interpolation filter 39, and the 0 value interpolation characteristic data as the data in the frequency region.
The convolution in the frequency region becomes the multiplication with a window function in the time region as a pass band of the interpolation filter 39. As a result, the filtering for the 0 value interpolation characteristic data in the interpolation filter 39 can be expressed in the form of the multiplication of (the data in the time region of) the 0 value interpolation characteristic data, and the window function as the pass band of the interpolation filter 39 in the time region.
In FIG. 7 (to which FIG. 10 as will be described later is also similar), the filtering for the 0 value interpolation characteristic data is expressed in the form of the multiplication of the 0 value interpolation characteristic data and (the window function corresponding to) the pass band of the interpolation filter 39.
In the 0 value interpolation characteristic data, the period of the multi-path which is repeated three times is Tu/3 sec. Therefore, the interpolation filter 39, for example, is configured as an LPF in which a band of −Tu/6 to +Tu/6 of the same band width as the period of Tu/3 sec. of the multi-path repeated three times is used as the pass band, thereby making it possible to extract the desired multi-path corresponding to the frequency direction interpolation characteristic data.
It is noted that the adjustment of the band width of the pass band of the interpolation filter 39 makes it possible to reduce the noise contained in the 0 value interpolation characteristic data (time direction interpolation characteristic data). This technique, for example, is described in Japanese Patent Laid-Open No. 2005-312027.
In addition, the filter center controlling portion (refer to FIG. 4) controls the position of the center of the pass band (the position of the filter center) in such a way that as shown in FIG. 7, the desired multi-path is contained in the pass band of the interpolation filter 39.
That is to say, as has been described, the filter center controlling portion 37 obtains the optimal position information on the position (optimal position) of the filter center where the signal quality of the OFDM signal (frequency region OFDM signal) after completion of the distortion correction is made best. Also, the filter center controlling portion 37 supplies the optimal position information representing the optimal position to the position adjusting portion 36 (refer to FIG. 4).
The position adjusting portion 36 adjusts (rotates) the phase of the time direction interpolation characteristic data in accordance with the optimal position information supplied thereto from the filter center controlling portion 37 in such a way that as shown in FIG. 7, the desired multi-path is contained in the pass band of the interpolation filter 39.
As a result, the position of the filter center (the center of the pass band) of the interpolation filter 39 is controlled in such a way that the desired multi-path is relatively contained in the pass band of the interpolation filter 39.
Here, when a part of the desired multi-path is not contained in the pass band of the interpolation filter 39, the estimation precision for the transmission characteristics in the transmission path characteristics estimating section 19 is deteriorated. As a result, the noise contained in the OFDM frequency region signal after completion of the distortion correction is increased to reduce the signal quality.
For this reason, the filter center controlling portion 37 obtains the position (optimal position) of the filter center where the signal quality of the OFDM frequency region signal after completion of the distortion correction is made best. Also, the filter center controlling portion 37 controls the position of the filter center of the interpolation filter 39 in such a way that the position of the filter center of the interpolation filter 39 is made to agree with the optimal position.
Now, in the case of the DVB-T or the ISDB-T, the arrangement pattern of the SPs is fixed to one kind of arrangement pattern. In addition thereto, the position of the transmission symbol of the TPS as an object for the obtaining of the signal quality, and the position of the transmission symbol of the TMCC/AC are also fixed. Therefore, there are carried out the estimation of the transmission path characteristics from the SPs, the distortion correction for the OFDM frequency region signal using the estimate values for the transmission path characteristics, and the calculations of the signal quality of the transmission symbol of the TPS of the OFDM frequency region after completion of the distortion correction, and the signal quality of the transmission symbol of the TMCC/AC. Thus, the position of the filter center of the interpolation filter 39 can be controlled in such a way that these signal qualities become best.
On the other hand, in the case of the DVB-T.2, as has been described, plural arrangement patterns (eight kinds of arrangement patterns) are determined as the arrangement parameters of the SPs. Also, any one of the plural arrangement patterns is selected, and the SPs are arranged in accordance with the arrangement pattern thus selected. Also, in the case of the DVB-T.2, the information on the arrangement pattern of the SPs is transmitted in a state of being specially contained in the OFDM signal.
In this case, the arrangement pattern of the SPs cannot be recognized until the information on the arrangement pattern of the SPs is decoded. As a result, the position of the filter center of the interpolation filter 39 becomes difficult to control in such a way that the signal quality becomes best, and thus the estimation precision for the transmission path characteristics is deteriorated.
That is to say, in the case of (a blue book of) the DVB-T.2, a frame called a T2 frame is defined, and the data is transmitted in T2 frames.
(The OFDM signal of) The T2 frame contains therein two kinds of Preamble signals called P1 and P2, respectively, and information necessary for the processing such as the decoding of the OFDM signal is contained in the preamble signals.
FIG. 8 is a diagram showing a format of the T2 frame.
An OFDM symbol of one P1 (hereinafter referred to as “a P1 symbol” as well), OFDM symbols of one or more P2 (hereinafter referred to as “P2 symbols” as well), OFDM symbols of one or more pieces of the data (Normal) (hereinafter referred to as “data symbols” as well), and OFDM symbols of necessary Frame Closing (FC) are contained in this order in the T2 frame.
Bits S1 and S2 are contained in P1 Signalling (P1).
The bits S1 and S2 contain therein information as to whether or not the frame is the T2 frame, and information as to whether or not the T2 frame and a Future Extension Frame (FEF) are mixed with each other in terms of the frame.
In addition, the bits S1 and S2 contain therein information as to in accordance with which of a Single Input Single Output (SISO) system and a Multiple Input, Single Output (MISO) system the OFDM symbols (the P2 symbols, the data symbols, and the OFDM symbol of the FC) other than the P1 symbol is transmitted. Also, the bits S1 and S2 contain therein an FFT size (the number of samples (transmission symbols) (sub-carriers) as an object for one FFT arithmetic operation) when the FFT arithmetic operation for the OFDM symbols other than the P1 symbol is carried out, and the like.
It is noted that six kinds of sizes: 1K, 2K, 4K, 8K, and 16K are regulated as the number of transmission symbols (sub-carriers) composing one OFDM symbol, in a word, as the FFT size.
However, although with regard to the OFDM symbols other than the P1 symbol, any of the six kinds of FFT sizes described above can be used, with regard to the P1 symbol, only 1K of the six kinds of FFT sizes described above is used.
In addition, in the P2 symbol, the data (Normal) symbol, and the OFDM symbol of the FC, the same value is adopted as the FFT size and the Guard Internal (GI) length.
Here, since the P1 symbol contains therein information on the transmission system, the FFT size, and the like which are necessary for demodulation of the P2 symbols, the P1 symbol needs to be demodulated in order to demodulate the P2 symbols.
L1 composed of L1pre Signalling (L1pre) and L1post Signalling (L1post) is contained in the P2.
The L1pre contains therein a parameter necessary to decode the L1post, and a parameter necessary to demodulate the data symbol (and the OFDM symbol of the FC).
That is to say, for example, information on a modulation system (such as the BPSK) for the L1post, and the like are contained as the parameters necessary to decode the L1post in the L1pre. In addition, for example, a pilot pattern (PP) representing the arrangement pattern of the transmission symbols of the pilot signals (SPs), information as to presence or absence of extension of the transmission band with which the OFDM signal is transmitted (Extended carrier mode or Normal carrier mode), information as to the number of OFDM symbols contained in one T2 frame (T2 frame length), and the like are contained as the parameters necessary to demodulate the data symbols in the L1pre.
Here, eight kinds of pilot patterns PP1 to PP8 are determined as the arrangement pattern of the transmission symbols of the SPs in the data symbols in the DVB-T.2. In the data symbols, the transmission symbols of the SPs are arranged in accordance with one of the eight kinds of pilot patterns PP1 to PP8. A pilot pattern as the arrangement pattern of the transmission symbols of the SPs of the data symbols is contained in the L1pre.
The L1post contains therein information necessary for the receiving apparatus to access a Physical Layer Pipe (PLP).
That is to say, information on a modulation system for the PLP, information on a size and a position of the PLP within the T2 frame, and the like are contained as information necessary to access the PLP in the L1post.
When the receiving apparatus detects and decodes the P1 symbol to estimate the G1 length, the receiving apparatus can demodulate the P2 symbols. In addition, when the P2 symbols are demodulated, the L1pre can be decoded. Also, when the L1pre is decoded, the L1post can be decoded. Also, after that, the data symbol (and the OFDM symbol of the FC) can be demodulated.
It is noted that although in FIG. 8, two P2 symbols are arranged in the T2 frame, in the DVB-T.2, the number of P2 symbols arranged in the T2 frame is determined from the size of the FFT of the P2 symbol (and the data symbol).
For example, when the FFT size of the P2 symbol is either 32K or 16K, one P2 symbol is arranged in the T2 frame. In addition, for example, when the FFT size of the P2 symbol is 8K, two P2 symbols are arranged in the T2 frame.
FIG. 9 is a diagram showing a structure of the P1 symbol.
The P1 symbol has 1K (=1,024) transmission symbols as effective symbols.
Also, the P1 symbol has the following cyclic structure. That is to say, a signal B1′ which is obtained by frequency-shifting a part B1 on the head side is copied on a preceding side (on a preceding side in terms of time) of the effective symbols. Also, a signal B2′ which is obtained by frequency-shifting a remaining part B2 of the effective symbols is copied on a back side (on a following side in terms of time) of the effective symbols.
As has been described, since the P1 symbol has the cyclic structure, the receiving apparatus carries out an arithmetic operation for the correlation of the OFDM signals, thereby making it possible to detect the P1 symbol.
Also, when the receiving apparatus detects and decodes the P1 symbol, and also estimates a G1 length, the receiving apparatus can demodulate the P2 symbol and also can decode the L1 (consisting of L1pre and L1post).
Therefore, when the receiving apparatus does not demodulate the P2 symbol, the receiving apparatus cannot decode the L1, and also cannot recognize the pilot pattern of the data symbols.
Also, when the receiving apparatus cannot recognize the pilot pattern of the data symbols, the receiving apparatus cannot control the position of the filter center of the interpolation filter 39 by using the data symbols.
From the above, in the receiving apparatus, the control for the position of the filter center (hereinafter referred to as “the filter center position control” as well) of the interpolation filter 39 (refer to FIG. 4) cannot be carried out until the L1 has been decoded to recognize (acquire) the pilot pattern of the data symbols.
Therefore, the receiving apparatus cannot carry out the filter center position control until the L1 has been decoded after the starting of the reception of the OFDM signal.